Anomaly diagnosis system for a nuclear reactor core

ABSTRACT

An anomaly diagnosis system for a nuclear reactor core comprises an anomaly detecting unit incorporated into a fuel assembly of the nuclear reactor core, and a transmitter-receiver provided outside the reactor vessel, which transmits a signal by wireless to the anomaly detecting unit and receives an echo signal generated by the anomaly detecting unit by wireless. When the anomaly detecting unit detects an anomaly in the nuclear reactor core, such as an anomalous temperature rise in the fuel assembly, the mode of the echo signal deviates from a reference signal mode, and then the transmitter-receiver detects the deviation of the mode of the echo signal from the reference signal mode and gives an anomaly detection signal to a plant protection system, and thereby the nuclear reactor can be shut down without delay.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an anomaly diagnosis system for anuclear reactor core, which gives an anomaly detection signal to acontrol rod driving mechanism or the like upon the detection of ananomaly in the nuclear reactor core, such as an excessive temperaturerise in the reactor core, to shut down the nuclear reactor.

2. Description of the Prior Art

When an anomalous condition occurs in the fuel system of a nuclearreactor, a control rod is inserted in the core for safety control suchas emergency shut down.

Conventionally, the anomalous reaction of the fuel in a nuclear reactoris diagnosed by detecting the flow rate and temperature of the coolantby a flow meter and a thermometer provided near the outlet of thecoolant within the reactor vessel and connected through a cable to anexternal diagnosis unit, and the neutron flux density of the fuelassembly by a neutron detector inserted in the gap of the fuel assemblyand connected through a cable to the external diagnosis unit.

Such a conventional diagnosis system, however, has the followingproblems.

First, the detection and warning of an anomalous condition by thedetector provided at the exit of the reactor vessel or in the gap of thefuel assemblies delays the implementation of safety countermeasures.

Secondly, it is impossible to easily change the detector provided nearthe coolant outlet of the reactor vessel or in the gap of the fuelassemblies.

Thirdly, it is difficult to extend a cable connecting the detector tothe outside reactor vessel and the deterioration of the cable reducesthe reliability of anomaly diagnosis.

Fourthly, when the detector is inserted in the gap of the fuelassemblies, the detected neutron flux density and the detectedtemperature of the fuel assembly are dependent on the position andcondition of the fuel assembly and are liable to deviate from truevalues, and thereby the reliability of anomaly diagnosis is reduced.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide ahighly reliable anomaly diagnosis system for a nuclear reactor core,eliminating the foregoing disadvantages of the conventional anomalydiagnosis system, capable of implementing safety countermeasures withoutan unacceptable delay, using detectors capable of being easily changed,eliminating difficulties in extending cables for anomaly detection, lessliable to cause the cables to deteriorate, and capable of obtainingcorrect detected values regardless of the position and condition of thefuel assembly.

To achieve the object of the invention, the present invention providesan anomaly diagnosis system for a nuclear reactor core, comprising: ananomaly detecting unit provided in the fuel assembly, the anomalydetecting unit being capable of generating an echo signal which isvaried upon the detection of an anomaly in the nuclear reactor core; anda transmitter receiver which gives a signal to the anomaly detectingunit to make the anomaly detecting unit provide an echo signal and givesan anomaly detection signal to a plant protection system upon thereception of an echo signal representing an anomaly in the nuclearreactor core from the anomaly detecting unit.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the general circuit constitution of ananomaly diagnosis system, in a first embodiment, according to thepresent invention;

FIG. 2 is a schematic sectional side elevation of a reactor vessel, inthe first embodiment, according to the present invention;

FIG. 3 is a longitudinal sectional view of a fuel assembly, in the firstembodiment, according to the present invention;

FIG. 4 is a circuit diagram of an anomaly diagnosis system, in a secondembodiment, according to the present invention;

FIG. 5 is a schematic illustration of an anomaly diagnosis system, in athird embodiment, according to the present invention;

FIG. 6 is a longitudinal sectional view of a fuel assembly shown in FIG.5, in the third embodiment, according to the present invention; and

FIG. 7 is a partial longitudinal section of a modification of the fuelassembly of FIG. 6, in another example of the third embodiment,according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT First Embodiment (FIGS.1, 2 and 3)

Referring to FIGS. 1 through 3, a plurality of fuel assemblies 2 aredisposed within a reactor vessel 1. As shown in FIG. 3, each fuelassembly 2 comprises a duct 3 and a plurality of fuel pins 4 containedin the duct 3. A coolant is introduced into the duct 3 through an inlet5 formed at the bottom of the fuel assembly 2 and exchanges heat withthe fuel pins 4 to cool the fuel pins 4 as the coolant flows toward anoutlet 6 formed at the upper end of the fuel assembly 2. An anomalydetecting unit comprises thermocouples 7a and 7b for detecting thetemperature of the coolant, and an echo capsule 8 connected to thethermocouples 7a and 7b. The thermocouples 7a and 7b are providedrespectively near the inlet 5 and outlet 6 of the fuel assembly 2.

As shown in FIG. 1, the echo capsule 8 is associated with an externaltransmitter-receiver 9 for wireless communication therebetween by usingradio waves. The external transmitter-receiver 9 transmits a radio waveto the echo capsule 8 and receives a radio wave echo of a frequency fromthe echo capsule 8. The normal frequency of radio wave echo provided bythe echo capsule 8 varies when the echo capsule 8 detects ar anomaly inthe nuclear reactor core. The echo capsule 8 comprises an oscillatingcircuit consisting essentially of an exciting coil 10, avariable-capacity diode 11, a capacitor 12, and a terminal assembly 13for connecting the echo capsule 8 to the thermocouples 7a and 7b. Thevariable-capacity diode 11 has a fixed electrostatic capacity Ccorresponding to the electromotive force V of the thermocouples 7a and7b, and the echo capsule 8 has a resonance frequency Hf corresponding tothe electrostatic capacity C of the variable-capacity diode 11.

When the normal frequency of the echo capsule 8 varies, the externaltransmitter-receiver 9 gives an anomaly detection signal through a cable14 to a plant protection system 15, for example, for a control roddriving mechanism.

In operation, the external transmitter-receiver 9 transmits a pulseradio wave of the resonance frequency Hf to excite the echo capsule 8.When no anomaly is found in the nuclear reactor core, the echo capsule 8transmits a radio wave echo representing a normal condition. Theexternal transmitter-receiver 9 receives the radio wave echo and countsthe frequency of the radio wave echo.

Suppose that an anomaly has occurred in the nuclear reactor core and thetemperature difference between the coolant at the inlet 5 and thecoolant at the outlet 6 has been caused to deviate from a referencetemperature difference, the difference in thermoelectromotive forcebetween the thermocouples 7a and 7b varies and the electrostaticcapacity of the variable-capacity diode 11 varies. Consequently, theresonance frequency of the echo capsule 8 varies and hence the frequencyof the radio wave echo varies. Upon the detection of frequency variationof the radio wave echo, the external transmitter-receiver 9 gives theanomaly detection signal through the cable 14 to the plant protectionsystem 15 for the control rod driving mechanism to shut down the nuclearreactor.

In case the echo capsule 8 malfunctions, the frequency of the radio waveecho is stopped. Therefore, the malfunction of the echo capsule 8 can bedetected through the detection of the stoppage of the frequency of theradio wave echo.

When the respective fuel assemblies 2 are provided with echo capsules 8respectively having different resonance frequencies, an anomalous fuelassembly 2 among the plurality of fuel assemblies 2 can be identified.

Second Embodiment (FIG. 4)

In the second embodiment, the echo capsule 8 has an oscillating circuitincorporating an oscillator having a frequency of normal mode ofvibration, such as a quartz oscillator 16, the frequency of normal modeof vibration varies according to temperature.

Third Embodiment (FIGS. 5, 6 and 7)

An anomaly diagnosis system, in a third embodiment, according to thepresent invention, has an external ultrasonic wave emitter-receiver 17provided outside a reactor vessel 1. An ultrasonic wave reflectingmember 18 as means for detecting an anomaly is provided on the top ofeach fuel assembly 2 so as to reflect an ultrasonic wave emitted fromthe ultrasonic wave emitter-receiver 17 toward a fixed point on theultrasonic wave emitter-receiver 17. The ultrasonic wave reflectingmember 18 is formed of a shape memory alloy. When the ultrasonic wavereflecting member 18 deforms when heated to a predetermined shape memoryrestoration temperature, and thereby the reflecting direction of theultrasonic wave reflecting member 18 is caused to change.

In a modification, the ultrasonic wave reflecting member 18 may beattached to one of the fuel pins 4 as shown in FIG. 7. The ultrasonicwave emitter-receiver 17 decides that an anomalous condition occurred inthe fuel assemblies 2 when the same is unable to receive the echoultrasonic wave.

In operation, all the ultrasonic wave reflecting members 18 reflect theultrasonic wave emitted from the ultrasonic wave emitter-receiver 7toward the ultrasonic wave emitter-receiver 17 as an echo ultrasonicwave. When the temperature of the coolant rises to the predeterminedshape memory restoration temperature due to an anomaly in the nuclearreactor core, the ultrasonic wave reflecting members 18 deform to changethe respective directions of reflection thereof. Consequently, theultrasonic wave emitter receiver 17 is unable to receive any echoultrasonic wave. Thus, the ultrasonic wave emitter-receiver 17 detectsthe anomaly and gives an anomaly detection signal to a plant protectionsystem 15.

When the ultrasonic wave reflecting member 18 is formed of a shapememory alloy containing a fissile material dispersed or sandwichedtherein, the temperature of the ultrasonic wave reflecting member 18 iscaused to rise to the shape memory restoration temperature by theanomalous increase of neutron flux density as well as the rise in thetemperature of the coolant.

It is also possible to identify an anomalous condition in the fuelassembly 2 by searching the fuel assembly 2 from which the ultrasonicwave emitter-receiver 17 is unable to receive the echo ultrasonic wave.

As apparent from the foregoing description, the anomaly diagnosis systemfor a nuclear reactor core, according to the present invention has thefollowing effects.

(1) The anomaly detecting unit incorporated into the fuel assemblydetects an anomaly in the fuel assemblies directly, and a signalindicating an anomaly is transmitted by wireless from the echo capsuleto the transmitter-receiver. Accordingly, the anomaly in the nuclearreactor core can immediately be detected and emergency countermeasuresfor shutting down the nuclear reactor can be implemented without delay.

(2) Incorporated into the fuel assembly, the anomaly detecting unit canbe taken outside the reactor vessel and can easily be changed.

(3) Since the anomaly detecting unit communicates with thetransmitter-receiver by wireless, the anomaly detecting unit and thetransmitter-receiver need not be interconnected by a cable, and hencethe anomaly diagnosis system eliminates difficulties in laying a cableand problems attributable to the deterioration of the cable and thereliability of anomaly diagnosis is improved.

(4) Since the diagnostic function of the anomaly diagnosis system is notaffected by the position and condition of the fuel assembly, the anomalydiagnosis system has high reliability.

(5) Even if the anomaly detecting unit should fail to function properly,the malfunction of the anomaly detecting unit can be found through thedetection of disappearance of the echo signal.

(6) Emergency countermeasures can immediately be implemented withoutdelay to shut down the nuclear reactor in case an anomaly occurs in thenuclear reactor core, and hence the higher burnup can be obtainedwithout entailing any danger to improve the fuel cost economy of thenuclear reactor.

Although the invention has been described in its preferred form with acertain degree of particularity, obviously many changes and variationsare possible therein. It is therefore to be understood that theinvention may be practiced otherwise than specifically described hereinwithout departing from the spirit and scope thereof.

What is claimed is:
 1. In combination, a nuclear reactor having areactor core with at least one fuel assembly, and an anomaly diagnosissystem, said system comprising:at least one anomaly detecting meansincluding an oscillating circuit and responsive to a radio wave forgenerating an echo signal which varies in response to an anomaly in thenuclear reactor core, said anomaly detecting means including sensors forsensing the condition of the nuclear reactor core, said anomalydetecting means being incorporated into said fuel assembly; and atransmitter-receiver means for transmitting said radio wave to theanomaly detecting means to make the anomaly detecting means generate theecho signal and for receiving the echo signal generated by the anomalydetecting means, said transmitter-receiver means including means forexamining the echo signal, and means for determining that an anomaly hasoccurred in the nuclear reactor core and for giving an anomaly detectionsignal to a plant protection system upon the detection of a deviation ofthe echo signal from a reference signal mode.
 2. The combinationaccording to claim 1, wherein the oscillating circuit comprises anexciting coil, a variable-capacity diode and a capacitor.
 3. Thecombination according to claim 1, wherein said fuel assembly includes acoolant inlet and a coolant outlet for permitting reactor coolant topass therethrough, and wherein the sensors are thermocouples for sensingthe temperature of the reactor coolant and are provided respectivelynear the coolant inlet and hear the coolant outlet of said fuelassembly.
 4. The combination according to claim 1, including a pluralityof said anomaly detecting means and said fuel assemblies, each saidoscillating circuit having a predetermined resonance frequency whichdefines the reference signal mode of the associated said echo signal,the fuel assemblies being provided respectively with said anomalydetecting means having respective said oscillating circuits which eachhave a different resonance frequency, whereby an anomalous fuel assemblyamong the plurality of fuel assemblies can be identified when theresonance frequency of the oscillating circuit incorporated in theanomalous fuel assembly varies.
 5. In combination, a nuclear reactorhaving a reactor core with at least one fuel assembly, and an anomalydiagnosis system, said system comprising:an anomaly detecting meansincluding an oscillating circuit and responsive to a radio wave forgenerating an echo signal which varies in response to an anomaly in thenuclear reactor core, said oscillating circuit including an oscillatorhaving a frequency of normal mode of vibration which varies depending ontemperature, said anomaly detecting means being incorporated into saidfuel assembly; and a transmitter-receiver means for transmitting saidradio wave to the anomaly detecting means to make the anomaly detectingmeans generated the echo signal and for receiving the echo signalgenerated by the anomaly detecting means, said transmitter-receivermeans including means for examining the echo signal, and means fordetermining that an anomaly has occurred in the nuclear reactor core andfor giving an anomaly detection signal to a plant protection system uponthe detection of a deviation of the echo signal from a reference signalmode.
 6. The combination according to claim 5, wherein the oscillator isa quartz oscillator.